A separator for a lithium-ion secondary battery is generally made by thin and porous insulating material which has not only good ion permeability and mechanical strength but also long-term stability regarding to chemical substances and chemical solvents. Therefore, the positive electrode and the negative electrode of the battery are separated by the insulating separator, and a short circuit caused by the contact of the two electrodes can be prevented. Meanwhile, because the separator is porous, Li-ions (lithium ions) may pass through the separator easily. In this way, a good ion conductivity between the two electrodes may be ensured. When a short circuit or a wrong connection happens externally of the battery, an abnormal large current is generated inside the battery and the temperature in the battery is increased. When the temperature reaches a predetermined value, the separator is fused by heat and then the porous structures of the separator may be closed. Then the current may be cut off, and the battery may stop operating. In this way, the safety of the battery may be improved. Therefore, the separator has an important influence on the lifespan of the battery. Especially in a high power battery capable of obtaining large power and large current density without voltage interrupt in a short time, a good performance of the battery may be obtained by optimizing the performance of the positive and negative materials. The separator of such high power battery may be as thin as possible. When the lithium-ion battery is in a large current condition, however, a large number of lithium dendrites may be formed and the separator may be pierced by the lithium dendrites, thus causing a short circuit in the battery, which may cause a security problem accordingly. Therefore, a separator with a good high-temperature stability is necessary. Then a stable high power battery with excellent performances can be obtained.
At present, the separator is mainly formed by porous organic polymer film. A typical organic separator includes polyethylene (PE), polypropylene (PP), and three-layer (polypropylene/polyethylene/polypropylene) composite membrane. Such organic polyolefin separators have certain disadvantages, such as a low melting point (for example, the melting point of PE is 130° C., the melting point of PP is 180° C.), a low thermo-stability, and a relative low chemical stability in the lithium battery system. In addition, as the separator is contacted with lithium or lithium intercalated graphite in the lithium battery, the polyolefin separator may be corroded gradually.
Recently, the separator is improved by coating a ceramic heat resistant layer on a surface of the composite separator, in which the ceramic heat resistant layer contains ceramic particles and a binder, and an organic solvent having a good wettability with a porous flexible substrate may be adopted as a solvent. The solvent includes N-methylpyrrolidone, N,N-Dimethyl Acrylamide, N,N-dimethylformamide and dimethyl sulfoxide. Then the structure stability, thermo-stability and safety of the separator may be improved to some extent. For example, US patent application publication No. US2005084761 discloses electrical separators and a process for making them. The process for producing such separator comprises applying to a sheet-like flexible substrate having a plurality of openings a coating on and in said substrate, the material of said substrate being selected from woven or non-woven electrically nonconductive fibers of polymers and/or natural fibers and said coating being a porous electrically insulating ceramic coating. The said substrate is selected from nonwovens of polymeric and/or natural fibers and said coating is a porous ceramic coating brought onto and into said substrate by applying to said substrate a suspension comprising at least one oxide of the metals selected from the group consisting of Al, Zr, Si, Ti, Y and mixtures thereof and a sol and heating one or more times to solidify said suspension on and in said substrate. The said adhesion promoter is selected from the group consisting of 3-amino-propyltriethoxysilane, 2-aminoethyl-3-amino-propyltri-methoxysilane, 3-glycidyloxy-trimethoxysilane, 3-meth-acryloyloxypropyltrimethoxysilane, vinyltriethoxysilane, vinyltrimethoxysilane, vinyltris(2-methoxyethoxy) silane and mixtures thereof. The said ceramic coating is at least one selected from the group consisting of BaTiO3, Al2O3, SiO2 and ZrO2.
The coating layer prepared according to the above-identified process has a relative low adhesion with the sheet-like flexible substrate. The ceramic layer is combined with the substrate just by the adhesive in the ceramic layer, and the bonding strength therebetween is relative low. Thus, particles of the coating layer are easy to fall off during processing and operating processes of the battery. Therefore, the high temperature resistance of the separator prepared by the above method is poor. The falling of the particles of the coating layer may cause performances of the separator to be uniform and may have bad effects on the consistency of the performance of the battery. Worse still, the transfer resistance to the lithium ions in the electrolyte may also be increased, which may be harmful to the fast charge-discharge of the battery. The lithium ions may be transferred onto the surface of the positive and/or negative electrode, which may affect the insertion and remove of the lithium ion. In addition, a pinhole which is possible to cause a short circuit in the battery may be formed in the separator. As can be concluded, performances and practical uses of the battery are seriously influenced.